Speed control system for d.c. motor

ABSTRACT

A control system is composed of a saturation amplifier delivering a positive or negative saturated voltage depending on the sign of a difference voltage between a command voltage and a negatively fed back speed-representing voltage of the motor to be controlled, a proportional and integrating amplifier receiving a difference voltage between the output of the saturation amplifier and a negatively fed back voltage representing the actual armature current through the motor as its input, and a power amplifier amplifying the output of the second amplifier for supplying the armature current for the motor. In other aspects of the invention, the first amplifier may be replaced by another proportional and integrating amplifier cooperating with a circuit portion prohibiting the integrating operation depending on the output of the replacing amplifier, or the power amplifier may be replaced by a DC current switching circuit employing an amplitude modulated pulse signal for switching the armature current of the motor as desired.

United States Patent {72] Inventors SadaakiNanai;

Nobuhiro Kyura, both of Fukuoka-ken, Japan 21 AppLNo. 851,945 [22] Filed Aug.21,l969 [45] Patented Aug. 10, 1971 [73] Assignee Kabushlki Keisha Ynskawa Denki Seisaltusho Fukoka-kenJapan [54] SPEED CONTROL SYSTEM FOR D.C. MOTOR 7 Claims, 13 Drawing Figs.

[52] U.S.Cl 318/327, 3l8/332,318/398 [5|] Int.Cl H02p5/l6 [50] FieldotSearch 3l8/326, 329,397,398,33l,332,327

[56] References Cited UNITED STATES PATENTS 3,284,688 11/1966 Black 318/326 3,383,578 5/1968 Lewis.... 318/326 3,458,791 7/1969 Boice 318/327 1 2 ll a 2 E 1 II INTEGRATING G 1A5) (9 MI) ANPLIFIER r-i -FILTER tun Primary Examiner-OrisL. Rader Assistant Examiner-Thomas Langer Attorney-Ward, McElhannon, Brooks and Fitzpatrick ABSTRACT: A control system is composed of a saturation amplifier delivering a positive or negative saturated voltage depending on the sign of a difference voltage between a command voltage and a negatively fed back speed-representing voltage of the motor to be controlled, a proportional and integrating amplifier receiving a difference voltage between the output of the saturation amplifier and a negatively fed back voltage representing the actual armature current through the motor as its input, and a power amplifier amplifying the output of the second amplifier for supplying the armature current for the motor. In other aspects of the invention, the first amplifier may be replaced by another proportional and integrating amplifier cooperating with a circuit portion prohibiting the integrating operation depending on the output of the replacing amplifier, or the power amplifier may be replaced by a DC current switching circuit employing an amplitude modulated pulse signal for switching the armature current of the motor as desired.

PATENIED AUG] 0 I97! I 3,599,063

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CURRENT REPRESENTING THE MOTOR TORQUE (A) 31 RECTANGULAR WAVE SOURCE PATENTED AUG 1 0 1911 SHEET 8 BF 8 SPEED CONTROL SYSTEM FOR D.C. MOTOR BACKGROUND OF THE INVENTION This invention relates to speed control systems for DC motors and more particularly to a system which affords not only precision control of the DC motor speed but also exact control of the motor starting and stopping operations in the normal running direction and the reverse running direction.

Although various control systems for DC motors have been disclosed heretofore, none of such control systems has yet exhibited satisfactory performance especially when the DC motor is employed, for instance, in a magnetic tape feeding mechanism in an electronic computer wherein an extremely high speed response is required.

SUMMARY OF THE INVENTION Therefore, the primary object of the present invention is to provide a novel control system for a DC motor which, with respect to such severe commands for normal or reversed running or for stopping of the motor, can afford constant acceleration and constant steady-state speed operation in either of the normal and reverse directions.

Another object of the invention is to provide a speed control system capable of controlling the speed of a DC motor in a wide speed range and also of effecting control with quick response to the above stated operational commands.

Still another object of the invention is to provide a novel control system for a DC motor which is capable of controlling the motor speed in a narrow deviation range despite variations in the load torque or an outside disturbance, with simultane' ous preservation of the above described wide control range and quick response characteristics.

An additional object of the invention is to provide a novel control system for a DC motor wherein a pulse train amplitude modulated by a command signal is employed for controlling a switching circuit which is provided in the DC power supplying circuit for the DC motor, whereby economical power consumption of the control system is achieved.

, These and other objects of the present invention have been achieved by a novel control system for a DC motor which comprises a saturation amplifier delivering a positive or negative saturated voltage depending on the sign of a difference voltage between a command voltage and a negatively fed back speed-representing voltage of the motor to be controlled, a proportional and integrating amplifier receiving a difference voltage between the output of the saturation amplifier and a negatively fed back voltage representing the actual armature current in the motor as its input, and a power amplifier amplifying the output of the second amplifier for supplying the armature current for the motor. In other aspects, of the invention, the first amplifier in the above described control system may be replaced by another proportional and integrating amplifier cooperating with a circuit portion prohibiting the integrating operation depending on the output of the replacing amplifier, or the power amplifier may be replaced by a DC current-switching circuit employing an amplitude-modulated pulse signal for switching the armature current of the motor as desired. As will be seen from the following description, the term proportional" as applied to an amplifier indicates that the amplifier output is proportional to the amplifier input; and the phrase proportional and integral" as applied to an amplifier indicates that the amplifier output is proportional to a time integral of the amplifier input.

The nature, principle, and utility of the invention will be more readily apparent from the following detailed description with respect to preferred embodiments thereof when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIGS. 1 and 2 are graphical representations indicating variations of quantities with time for a description of an optimum control system for a DC motor according to the present invention;

FIG. 3 is a block diagram showing a control system for a DC motor constituting a first embodiment of the present invention, wherein a saturable amplifier is employed in the input stage for the command voltage;

FIG. 4 is an expanded representation of the input-output characteristic of the saturable amplifier;

FIG. 5 is a circuit diagram of the first embodiment of the invention partly indicated in detail;

FIG. 6 is a circuit diagram showing a detailed example of organization of the power amplifier employed in the first embodiment of the present invention;

FIG. 7 shows oscillograms indicating the operational characteristics of the first embodiment of the invention;

FIG. 8 is a block diagram of a control system for a DC motor constituting a second embodiment of the present invention;

FIG. 9 is a circuit diagram of the second embodiment of the invention partly indicated in detail;

FIG. 10 is a detailed circuit diagram for a switching circuit which accomplishes ON-OFF control of the integrating operation of the proportional-integrating amplifier employed in the second embodiment of the invention;

FIG. 11 is a graphical representation indicating the variation of the speed variation ratio with the variation of the torque-representing current measured for the second embodiment of the invention;

FIG. 12 is a block diagram showing a third embodiment of the invention wherein a DC current switching circuit employing an amplitude-modulated pulse signal is utilized instead of the power amplifier supplying an electric power to the motor; and

FIG. 13 is a detailed circuit diagram showing the DC current switching circuit and the amplitude-modulated pulse generating circuit employed in the third embodiment of the invention.

DETAILED DESCRIPTION Referring first to FIGS. 1 and 2 showing optimum modes of control of a DC motor in accordance with the present invention, it is indicated that, upon application of a command voltage Ei of stepped form to the DC motor, the speed of the motor is varied from +N to N according to a predetermined sequence such as acceleration, constant speed running, and deceleration, or acceleration, deceleration, and reverse running, and the current flowing through the motor armature is thereby varied from +Ia to O and then to la at correspond ing time positions t through 17.

In FIG. 3 showing a first embodiment of the present invention, there are indicated, in the form of a block diagram, a saturable amplifier 1, a proportional and integrating amplifier (hereinafter called P.I. amplifier) 2, a transistor type power amplifier 3, a low inertia DC motor 4, a tachometer generator 5 coupled to the DC motor 4, a filter 6, and operators 1], 21. In the system of the above described organization, a voltage Etg obtained from the tachometer generator 5 and used as a feed back voltage related to the speed of the DC motor 4 is applied to the operator circuit 11 together with the input voltage Bi, and the resulting difference voltage A E is fed to the input of the saturable amplifier l. The amplifier 1 is, in itself, a switching amplifier, the input/output characteristic of which is indicated in FIG. 4 on a much enlarged scale.

The proportional amplification range of the amplifier is found to be an extremely narrow one (AE), narrower than a voltage corresponding to a desired precision (for instance, assuming that the desired precision is 1 percent and the feedback voltage from the tachometer generator is 1V, then the voltage corresponding to the desired precisionis l0 millivolt), whereby any harmful effect such as a ripple in'the voltage from the tachometer generator can be substantially eliminated. When the output voltage of the amplifier 1 is assumed to be Ii, and the voltage component of the armature current (:Ia) of the motor 4 is assumed to belo, the difference voltage :AI of the two voltages Ii, I is applied to the input side of the P.I. amplifier 2, and the power amplifier 3 is driven by the output voltage of the P.I. amplifier 2."The armature current In or Ia of the motor 4 is thus controlled.

Numeral 41 designates a resistor to pick up the voltage I0, and the filter 6 is employed for eliminating resonance frequencies which may be created at the time a mechanical resonance occurs in the tachometer generator portion.

Referring now to FIG. also showing the first embodiment of the present invention, there are indicated in detail the saturable amplifier 1, the P.I. amplifier 2, and the filter 6, all indicated in FIG. 3, and further and adjusting portion 7 of the control device. In the circuits, numeral 101 designates an input terminal to which the command voltage E1 is applied. Numerals 102 through 105, 601 through 603, 701 through 705, 201, and 202 designate resistors, numerals 108, 109, 604 through 606, and 203 designate capacitors, numerals 106, 107 designate Zener diodes, and numerals 100, 200 designate high-gain sign-converting amplifiers.

When the resistor 701 in the adjusting portion 7 is adjusted, the above described voltage difference AE can be regulated, and by the adjustment of the resistor 702, the saturated value of the output voltage Ii of the amplifier 1 can be regulated. Likewise, the integrating time-constant of the P.I. amplifier 2 can be regulated by adjusting the resistors 704. 705 in the adjusting portion 7. When the above described mechanical oscillation occurs, the harmful effect can be eliminated by somewhat expanding the voltage range AE corresponding to the linear portion of the characteristic of the amplifier 1.

Although, in the general form of the conventional control device of similar type, a negative feedback of a voltage corresponding to the armature current to the power amplifier preceding the DC motor and also another feedback of the voltage Elg corresponding to the motor speed outwardly to the above-mentioned feedback to a further preceding stage of the control circuit were provided, an armature current In such as that indicated in FIGS. 1 and 2 could never be obtained with respect to a step-formed command voltage. Furthermore, since the output voltage Ii was always required at the output side of the amplifier 1 even while the motor 4 was running at a constant speed, optimum control of a minimum response time of the DC motor could not be accomplished.

According to the present invention, in addition to the above described circuit components, an integrating amplifier having a reset time Tr is provided between the motor 4 and the amplifier 1, so that at P.I. operation is achieved with respect to the armature current Ia of the motor 4. For this reason, indicial response of the current Ia with respect tothe command input current li will have no attenuation, and a similar stepped output can be obtained with respect to a stepped input. Since a speed feedback is provided outwardly of the control system, the differential ofthe voltage will be E=Ei'S/(S+c), wherein, 5 is a Laplacean operator, and C is a constant. Accordingly, with respect to the stepped command voltage Ei=l S, an ultimate deviation of can be obtained. Since the amplifier l is a switching amplifier of continuously operable type, the response of the system can be made extremely rapid, whereby an optimum control system of fastest response can be realized.

In FIG. 6, a detailed connection diagram for the power amplifier 3 employed in this embodiment of the invention is indicated. In this power amplifier, there are included diodes 301 through 308, transistors 309 through 315, DC power sources 316 through 319, resistors 320 through 344, capacitors1345 il th'rough 348, an input terminal 40, and output terminals 42, 43. Since the output voltage of the amplifier is fed back from the output terminal 42 to the input terminal 40 through a resistor 344 as its minor loop, the voltage drop due to the resistors 335 through 337, 341 through 343 which are employed for stabilizing the thermal characteristics of the power transistors 313 and 315 can be eliminated. Furthermore, the disadvantageous effects caused by the internal resistances of the power sources 318 and 319 and ripples therein can be eliminated by the above-mentioned feedback, and quick frequency response of the power amplifier and linearity of the output characteristic can be attained.

In FIG. 7, there are indicated oscillograms of the output voltage (curve 1000) of the amplifier 3, the armature current (curve 2000), and the rotating speed of the motor 4 (curve 3000; minute oscillations on this curve are caused by the resonance of the tachometer generator 5 around a frequency of 1.68 KC and the rotating speed of the motor 4 is really a smooth curve), these curves being for the first embodiment of the present invention and being represented against the same abscissa of time.

A block diagram for the second embodiment of the invention is shown in FIG. 8. In this embodiment, instead of the switching amplifier l of the first embodiment, a proportional and integrating amplifier 20 is provided, and an analog-switch 8 operated by the output of the proportional and integrating amplifier transfers the operational characteristic of the PI amplifier between a proportional integrating mode and an integrating mode. More specifically, at the starting or stopping operation of the motor wherein a limitation of the armature current is required, the control system is operated in the proportional mode (P-mode), and in the rest of time, the control system is operated in the proportional and integrating mode (P-I mode) by means of the analog-switch 8 provided at a preceding stage of the control system. Thus, the operational characteristic of the control system and its load characteristic in the steady state can be simultaneously improved.

In the above described operation, since the P-I mode transferring operation is performed by its own output signal or the value of the armature current of the motor 4, a compensation for equalizing the full load speed can be carried out throughout the whole speed range of the motor 4, and, furthermore, a minimum-period-control being attained for the starting and stopping control of the motor 4, the response speed thereof may also be made fast as in the case of the first embodiment of the invention.

Now considering a condition wherein the load torque (or the armature current) is varied in the first example, the above mentioned switching amplifier 1 has a linear portion in the characteristic as shown in FIG. 4, and as a result steady-state deviation determined by the amplification factor in such a linear portion is created each time the variation in the load torque occurs. Since the deviation is decreased along with an increase in the amplification factor, the deviation may be considerably minimized in the first example. Theoretically, however, this value cannot be brought to zero, and a deviation of less than approximately 0.1 percent is inevitably involved in the system of the first example. For the improvement on this point, one more stage of proportional integrating element is provided before the P.I. amplifier 2 so that the stepwise variation of the load torque may be thereby neglected in the steady state.

When a proportional and integrating element is employed as a compensating element for the stepwise variation of the load torque, the steady-state deviation can be theoretically nullified. However, the problem is that a saturating characteristic should be imparted to the proportional and integrating element for the purpose of maintaining the command current at a constant value and that, when the compensating element operates as a proportional and integrating element having a saturation characteristic, the time instant at which the operational point leaves the saturating region is delayed due to the charging current flowing into the feedback capacitor in the amplifier during its saturating period, and, for this reason, the response at the time of starting and stopping of the motor 4 is seriously impaired.

To prevent this deterioration of response and, furthermore, to improve the steady-state characteristic of the control system, the proportional-integrating element is made operable in either the proportional mode or in the'integrating mode, and switching of these operational modes is carried out depending on the command current value applied to the motor. When the command current value is lower than a value determined from the accelerating current and the rated current of the motor 4 (the accelerating current 2( rated current) the element 20 operates as a proportional and integrating element, and, when the command current value is higher than the above-mentioned value determined from the accelerating current and the rated current of the motor 4, the element 20 operates as a proportional element partly saturated.

In FIG. 9, there is indicated the second embodiment of the invention partly shown in great detail. In the drawing, elements similar to or corresponding to those in the first embodirnent of the invention are designated by like reference numerals. Numerals 706 and 707 designate resistors, and numeral 109 designates a capacitor so arranged as to be short circuited when the output of an amplifier 100 in the element 20, that is, the input of the analog switch 8 (which is proportional to the armature current of the motor 4) falls within an amplitude around a central value of zero, and the amplitude is predetermined by the resistor 707 acting as a regulator.

In FIG. 10, there is indicated a detailed connection of the analog switch 8 which comprises resistors 801 through 817, transistors 818 through 823, capacitors 824 through 826, diodes 827 through 832, and a temperature compensating thermistor 833. With these circuit components, a synchronized switching circuit consisting mainly of a bridge circuit made up of the diodes 829 through 832 is organized.

When a signal exceeding a predetermined value arrives at the input terminal resistance 801, the output voltages of the transistors 818 through 820 are raised to higher levels as desired, which in turn elevate the emitter potential of the transistor 821 (or the junction point 834), whereby the transistor 822 is brought into off state, and the transistor 823 is supplied with a low voltage from the power source -B (-l8v.) As a result, the potential of the junction point 835 is lowered, and a current flows from a point 834 to a point 835. Thus a voltage across the capacitor 109 connected to a bridge circuit of the diodes 829 through 832 is short circuited through the power source (-3 and +B), and, thus, the element 20 does not operate in the integrating mode but operates in a quick response proportional mode.

On the other hand, when the signal applied to the resistor 801 is less than a predetermined value, the transistor 822 is brought into ON state, thereby elevating the potential of the junction point 835 and lowering the potential of the junction point 834', and the bright circuit consisting of the diodes 829 through 832 blocks the current to flow from the point 834 to the point 835, whereby the capacitor 109 performs its integrating function.

Experimental values obtained by the second embodiment of the invention are indicated in FIG. 11. As is apparent from the drawing, the speed variation for the running speeds of the motor ofilOOO r.p.m. is limited to less than 0.003 percent. In this system, the P.l. control region of the element 20 is set beforehand to a desired value in consideration of the loading range of the motor 4 employing the regulator portion 707.

In FIG. 12, there is indicated a third embodiment of the present invention, wherein a DC power supplying device employing a pulse amplitude-modulation is used instead of the power amplifier 3 in the second example shown in FIG. 9, which is constructed in the form of a transistor DC amplifier as shown in FIG. 6. in this example; there are included a magnetic amplifier 30, a rectangularwave generating source 31, a transistor switching circuit 33, and a driving circuit 32 placed at the preceding stage of the circuit 33, and the supply voltage E0 for the motor 4 is fed back to an operator 22.

The third embodiment of the invention is illustrated in more detail in FIG. 13 in which are included resistors 3001 through 3014, transistors 3015 and 3016, diodes 3017 through 3020, center-tap-doubler-type magnetic amplifiers IMA and 2MA, other resistors 3201 through 3210, other transistors 3211 through 3214, power transistors 3301 and 3302, inductances 3303 and 3304, other diodes 3305 and 3306, still another resistor 3307, and a capacitor 3308, and a frequency of 8 kc. is employed for the rectangular wave generated from the source 31 (generally this frequency may be increased up to kc.) In this example, the inductances 3303 and 3304 act as current limiting reactors for a circulating current which flows through the transistors 3301 and 3302 when each of the transistors conducts, and the inductances 3303 and 3304 also act as smoothing reactors for the DC modulated current (la or -la).

The diodes 3305 and 3306 constitute electrical valve members which form a circulating path starting from the inductance 3303, through inductance 3304, diode 3306, +8, -B, diode 3305, and again to inductance 3303, whereby the electrical energy stored in the inductances 3303 and 3304 by the circulating currents passing through the transistors 3301 and 3302 when these transistors are conducting is circulated back to the power source at the time the transistors 3301 and 3302 are both brought into OFF state, Since twice the power source voltage is applied to either of the transistors 3301, 3302 when these transistors are brought into OFF state, a full bridgetype transistor switching circuit is preferably employed instead of the semibridge-type switching circuit as shown in FIG. 13 if the power source voltage (1B) is considerably high or if the accelerating period of the motor 4 is excessively long.

By means of the third embodiment ofthe invention, a motor 4 of far larger DC power capacity than in the cases of the first and the second examples can be used, and the application range of the present invention is thereby much expanded.

As is apparent from the above description, according to the present invention, an excellent speed control system for a DC motor, having minimal response time, wide field of application, and the least speed variation ratio can be provided.

We claim:

1. A speed control system for DC motor, said system comprising, generator means for producing first negative feedback voltages whose magnitude and polarity correspond, respectively, to the speed and direction of rotation of said motor, armature current sensing means for producing second negative feedback voltages whose magnitude and polarity correspond, respectively, to the magnitude and direction of armature current in said motor, a command input terminal for receiving command input signal voltages whose magnitude and polarity correspond, respectively, to desired motor speed and direction, a first amplifier connected to produce first amplif er voltages of a predetermined magnitude when the first negative feedback and the input signal voltages differ, the polarity of said first amplifier voltages corresponding to the direction of voltage difference, a second amplifier connected to produce second amplifier voltages which correspond in magnitude and direction to the difference between an integral of said first amplifier voltages and said second negative feedback voltages and a power amplifier for amplifying said second amplifier voltages and for applying armature current to said motor corresponding to the thus amplified voltages.

2. A speed control system according to claim 1 wherein said power amplifier is a transistor type DC amplifier having temperature stabilizing resistors; and wherein said power amplifier includes negative feedback circuit means comprising a resistive connection between the amplifier output and input whereby voltage drops across the temperature stabilizing resistors are eliminated.

3. A speed control system for a DC motor, said system com prising, generator means for producing first negative feedback voltages whose magnitude and polarity correspond, respectively, to the speed and direction of rotation of said motor, armature current sensing means for producing second negative feedback voltages Whose magnitude and nnlaritu anama...

respectively, to the magnitude and direction of armature current in said motor, a command input terminal for receiving command input signal voltages whose magnitude and polarity correspond, respectively, to desired motor speed and direction, a first amplifier connected to produce first amplifier voltages which correspond in magnitude and direction to the difference between the first negative feedback voltages and the input signal voltages, a second amplifier connected to produce second amplifier voltages which correspond in magnitude and direction to the difference between the integral of said first amplifier voltages and for applying armature current to said motor corresponding to the thus amplified voltages,

signal integrating means switchable into and out of the signal flow path of said first amplifier and switch means connected to switch said signal integrating means out of the signal flow path nectingithe AC terminals of the bridge circuit across said capacitor and transistor switch means arranged to connect the DC terminals of the bridge circuit across a DC power source in response to voltages in excess of said first predetermined magnitude.

6. A speed control system according to claim 5 wherein said power amplifier comprises a direct current switching circuit responsive to amplitude modulated pulse signals to switch on and off a DC power source to said motor, wherein there is provided a further negative feedback means connected to feedback the motor supply voltage to the output of said second amplifier to modify same and wherein there are provided pulse modulation means to produce amplitude modulated pulse signals for said power amplifier in response to the modified output of said second amplifier.

7. A speed control system according to claim 6 wherein said direct current switching circuit comprises a rectangular wave source, wherein said pulse modulation means comprises a pair of center tapped doubler magnetic amplifiers, and wherein said power amplifier further comprises a transistor type DC amplifier connected to amplify the output of said pulse modulation circuit, and a bridge circuit formed by a plurality of power transistors connected to be switched on and off by outputs from said DC amplifier. 

1. A speed control system for a DC motor, said system comprising, generator means for producing first negative feedback voltages whose magnitude and polarity correspond, respectively, to the speed and direction of rotation of said motor, armature current sensing means for producing second negative feedback voltages whose magnitude and polarity correspond, respectively, to the magnitude and direction of armature current in said motor, a command input terminal for receiving command input signal voltages whose magnitude and polarity correspond, respectively, to desired motor speed and direction, a first amplifier connected to produce first amplifier voltages of a predetermined magnitude when the first negative feedback and the input signal voltages differ, the polarity of said first amplifier voltages corresponding to the direction of voltage difference, a second amplifier connected to produce second amplifier voltages which correspond in magnitude and direction to the difference between an integral of said first amplifier voltages and said second negative feedback voltages and a power amplifier for amplifying said second amplifier voltages and for applying armature current to said motor corresponding to the thus amplified voltages.
 2. A speed control system according to claim 1 wherein said power amplifier is a transistor type DC amplifier having temperature stabilizing resistors; and wherein said power amplifier includes negative feedback circuit means comprising a resistive connection between the amplifier output and input whereby voltage drops across the temperature stabilizing resistors are eliminated.
 3. A speed control system for a DC motor, said system comprising, generator means for producing first negative feedback voltages whose magnitude and polarity correspond, respectively, to the speed and direction of rotation of said motor, armature current sensing means for producing second negative feedback voltages whose magnitude and polarity correspond, respectively, to the magnitude and direction of armature current in said motor, a command input terminal for receiving command input signal voltages whose magnitude and polarity correspond, respectively, to desired motor speed and direction, a first amplifier connected to produce first amplifier voltages which correspond in magnitude and direction to the difference between the first negative feedback voltages and the input signal voltages, a second amplifier connected to produce second amplifier voltages which correspond in magnitude and direction to the difference between the integral of said first amplifier voltages and said second negative feedback voltages, a power amplifier for amplifying said second amplifier voltages and for applying armature current to said motor corresponding to the thus amplified voltages, signal integrating means switchable into and out of the signal flow path of said first amplifier and switch means connected to switch said signal integrating means out of the signal flow path of said first amplifier whenever either said second negative feedback voltages or the output of said first amplifier exceeds a predetermined magnitude.
 4. A speed control system according to claim 3 wherein said signal integrating means includes a capacitor and wherein said switch means comprises an electronic switch responsive to outpuT voltages from said first amplifier in excess of said predetermined magnitude to short circuit said capacitor.
 5. A speed control system according to claim 4 wherein said electronic switch comprises a diode bridge circuit, means connecting the AC terminals of the bridge circuit across said capacitor and transistor switch means arranged to connect the DC terminals of the bridge circuit across a DC power source in response to voltages in excess of said first predetermined magnitude.
 6. A speed control system according to claim 5 wherein said power amplifier comprises a direct current switching circuit responsive to amplitude modulated pulse signals to switch on and off a DC power source to said motor, wherein there is provided a further negative feedback means connected to feedback the motor supply voltage to the output of said second amplifier to modify same and wherein there are provided pulse modulation means to produce amplitude modulated pulse signals for said power amplifier in response to the modified output of said second amplifier.
 7. A speed control system according to claim 6 wherein said direct current switching circuit comprises a rectangular wave source, wherein said pulse modulation means comprises a pair of center tapped doubler magnetic amplifiers, and wherein said power amplifier further comprises a transistor type DC amplifier connected to amplify the output of said pulse modulation circuit, and a bridge circuit formed by a plurality of power transistors connected to be switched on and off by outputs from said DC amplifier. 